The present invention pertains to the radio receiver art and, more particularly, to a means for, and a method of aligning a radio receiver.
The radio receiver art has developed several techniques for the aligning of radio receivers. Alignment is a process whereby various stages of the receiver are properly tuned to the desired frequency. For example, when a modulated signal of predetermined carrier frequency is applied to the radio frequency input stage of a receiver, the local oscillator stage, tuned intermediate frequency stage, and detector stage must all be adjusted to maximally reproduced the signal at the (or demodulator in FM radio receiver) detector output.
A typical alignment procedure known in the prior art is as follows. A predetermined frequency signal is applied to the radio frequency input, or antenna connection, of the receiver being aligned. The local oscillator is adjusted such that the output from the mixer is the input signal converted to the receivers intermediate frequency which, in a common FM radio is 10.7 MHz. The intermediate frequency stage is then adjusted such that the alignment signal is directly in the center of its passband. For receivers employing a fixed tuned IF stage, however, the receiver may be misaligned due to the center band of the filter being at other than the derived intermediate frequency. This results in misalignment. Finally, a DC tuning meter is connected to the output from the detector stage and the detector is adjusted such that the output reading is zero volts DC. Also, due to component value drifts caused by aging and thermal effects, a reading of zero volts DC may, in fact, correspond to a misaligned receiver.
The above described prior art alignment procedure suffers from numerous disadvantages. Firstly, it is a complicated and tedious procedure, not generally suited to efficient manufacturing operation. Also, for receivers employing fixed intermediate frequency tuned stages, the above procedure does not provide optimum alignment. In addition, it is desirable for purposes such as field alignment to use a zero centered DC tuning meter. For radio designs utilizing a quadrature detector, a non zero DC output signal appears at the detector when the detector is properly aligned. This fact precludes the use of zero centered DC tuning indicators when used in the prior art aligning procedure.